Commercial aromatic nitration processes are generally operated with the mixed acid system comprising sulfuric and nitric acids. A substantial amount of nitric acid is typically lost in an economically and environmentally unsound manner in the spent acid of nitration. For example, conventional dinitrotoluene processes comprise reacting toluene in a first nitration stage to form mononitrotoluene and separating the organic product from the aqueous spent acid phase. The nitric acid in the spent acid phase is usually removed or recovered so that the remaining sulfuric acid can be reconcentrated and recycled. The crude mononitrotoluenes are then nitrated with fresh nitrating acid in a second nitration stage. The dinitrotoluene product is then separated from the aqueous spent acid phase which is recycled to a nitration stage.
Ideally, the nitric acid in the aqueous spent acid of the mononitration stage should be recovered with a minimum of processing and with no risk of discoloration (charring) of the spent acid, i.e. the formation of what is known in the art as black spent acid. One such method involves the extraction of the spent acid with the aromatic hydrocarbon being nitrated in the process. This method removes the excess nitric acid from the spent acid in the form of the mononitro derivative of the aromatic hydrocarbon. Specifically, in a dinitrotoluene process, toluene is used to extract the spent acid to recover the nitric acid as mononitrotoluene in a post-reactor. It is recognized that extremely tight control over the stoichiometric reaction of the aromatic hydrocarbon is necessary to prevent formation of sulfuric acid soluble color bodies which render the final denitrified sulfuric acid black or dark red. It is believed that minor amounts of nitrous acid in the spent acid is responsible for the formation of the sulfuric acid soluble color bodies. The presence of nitric acid hinders or prevents this reaction. Thus, upon the depletion of the nitric acid when an excess of aromatic hydrocarbon is used to extract the spent acid phase in a denitrator, the nitrous acid reacts with the aromatic compounds to generate black spent acid. Such "charred" or black spent acid is unacceptable in the industry particularly if the denitrified spent acid is sent to a phosphate producer.
In another method, treatment of the spent acid of nitration with a variety of oxidizing or reducing agents which destroy the nitrous acid present in the spent acid prior to extraction with the aromatic hydrocarbon will prevent the formation of discolored sulfuric acid. The cost of these reagents precludes their use in the industry.
U.S. Pat. Nos. 2,773,911 and 2,849,497 are representative of the art disclosing the extraction/reaction of the nitric acid in the spent acid of nitration by contact with an aromatic hydrocarbon.
U.S. Pat. No. 4,257,986 discloses an improvement in a process for the manufacture of a nitroaromatic compounds produced by the mixed acid nitration method. The improvement resides in the refining of the aqueous acid mixture which comprises (a) contacting the spent mixed acid with an oxidizing or a reducing agent under conditions effective for removing contaminant nitrous acid, (b) contacting the aqueous spent acid mixture in step (a) with feed aromatic compound to remove contaminant organics and residual nitric acid and then, if necessary, (c) contacting the remaining nitric acid mixture with sufficient oxidizing agent under oxidizing conditions to remove residual organic components.
U.S. Pat. No. 4,496,782 assigned to the assignee of the present application discloses a method for denitrifying the aqueous spent acid of mononitration in which the nitric acid in the aqueous spent acid is recovered by adiabatically reacting greater than a stoichiometric amount of a mononitroaromatic hydrocarbon with the aqueous spent acid which has been fortified to a nitric acid concentration of at least about 2 wt%.